the mac version will be going though a 3rd party in co-operation with SE, if you buy the windows you ONLY have access to the windows version if you buy the MAC version you will only have access to the mac version

as for steam it is very rare they make you buy both but it is unknown how that will play out.


in short if you want to play on both windows and mac you need to buy the game twice one for windows and one for mac.